Privileged access to parliament and its legislators is paradoxically both at the heart of most of the UK's political scandals and a fact of daily life in Westminster.

Whether it is the cash-for-questions scandals that plagued the Conservative government in the 90s, Labour's reliance on millionaire donors during its time in power, or questions over Liam Fox's close friendship with defence lobbyist Adam Werritty, the issue of how close parliamentarians get to vested interests remains thorny.

At the height of the furore over MPs' expenses, David Cameron, then leader of the opposition, warned that lobbying was the "next big scandal waiting to happen".

The Guardian's revelation that more than 170 parliamentary passes have been handed out to lobbyists, campaign group directors and other outside interests by members of the House of Lords shows how far the government has still to go to reform access to parliament's corridors.

Although there is no suggestion that any rules have been broken, peers have given parliamentary passes to the director of government affairs for BP, the political director of the Taxpayers' Alliance, to several defence lobbyists, and to four members of the Christian group Care.

Unlike their colleagues in the Commons, lords do not receive specific staffing allowances and so require fewer passes for researchers and other core staff. As a result, 281 of the 646 people issued passes by peers declare some form of outside interest.

By contrast, an earlier Guardian investigation found only about one in five MPs' researchers declared outside interests, though staff from PricewaterhouseCoopers, KPMG, Triton and others were all issued passes through this route.

Lord Campbell-Savours, who moved from the Commons to the upper chamber in 2001 and gave evidence to the Nolan committee on transparency, said the presence of lobbyists had become increasingly obvious in the House of Lords.

"Tightening up of the rules in the Commons means there is a risk that lobbyists could use the Lords to influence government policy in the future," Campbell-Savours said.

"Once they have a House of Lords parliamentary pass they would not have to go through security, which is a lengthy exercise.

"Instead, they could walk up to the committee rooms, Portcullis House and many of parliament's facilities. These are places where ministers and members meet and could be glad-handed and cajoled," he added.

The former Labour MP said he did not believe peers had been paid directly for offering these passes, but the relationship with a member of staff could lead to a blurring of the lines between a researcher's roles. "The moment that a peer hands over a pass and is promised some help in running their office, the relationship changes," he said.

"I have no objection to charities being passholders as long as the activities relate only to charities. The moment there is a pecuniary benefit or a secretarial support benefit for a member's other activities, I regard that as unacceptable."

As peers are appointed for life, they are less susceptible to public pressure over links to lobbyists than their counterparts in the Commons. This means passes are granted to members of some controversial causes, including to Edward Lord, the deputy chairman of the European Azerbaijan Society, which exists to "promote Azerbaijan to international audiences". Azerbaijan is consistently rated as "not free" in the annual Freedom House survey of international human rights.

The strong influence of the Church of England on the chamber is also reflected in the register of passholders, with members of numerous evangelical groups being granted passes by the bishops and their allies sitting in the house.

These include a pass issued by the bishop of Manchester to Dr Donald Horrocks, the head of public affairs for the Evangelical Alliance; two passes for Christian public affairs officers from the bishop of Chester; and a pass from Lady Berridge for Colin Bloom, the director of the Conservative Christian Fellowship.

Paul Lennon, parliamentary adviser for the anti-abortion group the Society for the Protection of Unborn Children, also has parliamentary access, granted by Lady Masham of Ilton.

The parliamentary registers have regularly come under fire from transparency campaigners as requiring only minimal disclosure of conflicts of interest by parliamentarians and their staff, with more than 40 cases on the Lords register alone where the precise nature of a passholder's outside work could not be determined.

Even with these lax rules, sometimes interests are not declared promptly.

One passholder who is registered as a peer's staff member admitted that he had failed to inform the house authorities that his job, and declarable interests, had changed.

House of Lords rules state that passholders are expected to notify the registrar of any change within 28 days.

Asheem Singh, a former Conservative party researcher, applied for a pass last year as a member of staff for Lord Bates, the Conservative peer.

At that point he declared his role as being the deputy director of the David Cameron-backed thinktank Respublica that was established by Philip Blond.

Singh left Respublica in June and started working as policy director for a not-for-profit charitable trust called Impetus, whose donors include Isis Private Equity, Barclays Capital, Deutsche Bank, CVC Capital Partners and Warburg Pincus. However, he is still registered on the House of Lords register as a Respublica employee.

Singh said he would correct his entry, and added that he had hardly used the pass, if at all, since joining Impetus. "If I have broken the rules, I am very happy to contact the authorities. I don't want to use the pass any more. I thought it had lapsed," he said.

Lords and their lobbyists

• Lord Moonie sponsored a pass for Robin Ashby, the director general of the UK Defence Forum, who was once stripped of his parliamentary pass after press interest.

• Lord Howard of Rising sponsored a pass for Richard Ritchie, director of government affairs for BP.

• Lord Black of Brentwood sponsored a pass for Edward Taylor, the head of public affairs for Telegraph Media Group.

• Lord Flight sponsored a pass for Jonathan Isaby, political director of the Taxpayers' Alliance.